Vocabulary: what does it mean for an equation to express y in terms of x?

To express y in terms of x is to find an expression that relates the value of x with y. This isto write a model, function, expression or formula that permits to input a value for x and the outcome will be the value of y. .For example if you have y = 2x, y is in terms of x and you can input different values of x to obtain corresponding values (outpus) of y.